Sudan on Thursday released the first detailed results of a census vital to elections due next year, but the figures were dismissed by former southern rebels who said the southerners' total was too low.

Any lengthy dispute over the census count risks delaying Sudan's first democratic election for more than 20 years, as well as a referendum on southern independence -- both centrepieces in a frayed north-south peace deal.

The census figures showed a total population of 39.15 million, with 30.89 million or 79 percent living in the mainly Muslim north, and 8.26 million or 21 per cent living in the predominantly Christian and animist south.

The south's dominant Sudan People's Liberation Movement (SPLM) accused northern politicians of manipulating the figures and under-counting the number of southerners both in the south and Khartoum.

Northern and southern Sudan fought each other for more than two decades in a civil war that was fuelled by a mixture of religion, ideology, oil and ethnicity. The conflict, separate from fighting in Sudan's western Darfur region that flared in 2003, ended in the 2005 peace deal which set up a north-south coalition government.

The census figures are supposed to define the constituencies for the election due in February 2010 before the referendum on southern independence scheduled the following year. They should also determine the share of power and wealth -- including oil -- between the north and the south.

OIL REVENUES

Under the peace deal, any proof the south had less than a third of Sudan's total population would mean recalculating how much oil revenue would flow to the region's capital Juba.

"We believe in many ways this census is politically motivated and designed," senior SPLM official Yasir Arman told Reuters in comments critical of the north's dominant National Congress Party (NCP). "It is clear the only elections the NCP are ready for are unfair and unfree elections."

Arman said earlier counts had shown southerners made up a third of Sudan's total population, not the fifth recorded in Thursday's figures. He said there was also a suspiciously sharp jump in the population of South Darfur state, part of northern Sudan.

Arman said Sudan would now have to find another, fairer way to work out the borders of constituencies and power-sharing between the two sides, in the run up to the election.

"We have to have a new voter registration. The constituencies should be divided according to the voter registration," he said.

No one was immediately available to comment from the NCP, led by Sudanese president Omar Hassan al-Bashir.

The census, carried out in April and May 2008, was hit by boycotts, demonstrations and lost questionnaires. More than 100 northern monitors were expelled from the south and Darfur rebels said they kidnapped 13 staff.